It is widely agreed that knowledge of an athlete's power output can be used to determine an athlete's overall performance. For example, knowing the power to weight ratio can determine how fast a cyclist can climb a given slope. Or in the case of a flat course time trial, the rider with the highest sustained power output is the likely winner. Aside from the ability to predict performance, knowledge of power output can be used to efficiently and effectively tailor an appropriate training program for that athlete with the ultimate goal being to increase their sustained power output. An effective training program balances the effort intensity, duration, and recovery period in order to continually improve power output and thereby overall fitness level. In addition, for a competitive athlete, an effective training program also includes a scheduled racing program to ensure that the athlete has peak performance for their targeted races. Such precise knowledge and control over an athlete's fitness level is achievable with power output measurements.
At present, commercially available cycling power meters are extremely expensive, making them out of reach for most people who would otherwise directly benefit from a more efficient and effective training program. These power meters rely on traditional strain gauge technology which although reliable, can be costly to manufacture, calibrate, must be temperature compensated, and require precise signal conditioning electronics.
Strain gauges measure deformation by means of changes in resistance to wire traces bonded to the surface of interest. This bonding, if not done properly, can eventually fail, requiring costly repairs at the factory or even discarding of the product altogether. As the deformation occurs, these bonded wire traces are stretched or compressed thereby changing their resistance. Very often these deformations are in the millistrain range and result in extremely small changes in resistance of the bonded wire traces. In order to precisely measure these small changes in resistance, highly regulated, low noise voltage or current supplies are needed. Such small changes in resistance, coupled with the fact that the output signal is proportional to the excitation voltage, the resulting output signal can be extremely small and requires additional amplification to be measured. Amplification of the signal will also amplify the inherent noise in the signal. As such, great emphasis must be placed to minimize or eliminate the sources of noise in the system.
Since strain gauges rely on changes in resistance to bonded wire traces, they are prone to ambient temperature variations which lead to noise and drift in the signal if not properly compensated for. There are various methods to compensate for temperature but they involve the placement of additional strain gauges, thereby increasing the power consumption of the final circuit. This increased power consumption has implications for portable power meters such those mounted on a bicycle.
Commercially available power meters contain electronics that require some sort of battery to provide electrical power. These batteries must be periodically replaced or in the case of some be exchanged by returning the product to the factory. Battery life is a critical issue for these products since the electronics not only measure the forces, but typically also have on board radios that transmit this information wirelessly thereby further increasing the current draw, beyond what is needed to measure strain. As a result, current consumption can be large and drain the battery, thereby requiring frequent replacement. At present, there are no known rechargeable power meters commercially available.